This soul is not obtained after defeating King Vendrick, but is instead obtained by returning to the Shrine of Amana after defeating him. Shortly after the Crumbled Ruins bonfire there is a submerged stone path in the water leading to the left of the cavern. The path ends at a wooden door behind some vines which will only open if the player is in human form and has already defeated King Vendrick. Inside is the Soul of the King and the King's Set.